This video is called The World of Seahorses.
From the Journal of ZooLogy:
Sounds produced by the longsnout seahorse: a study of their structure and functions
Seahorses are known to produce sounds in different behavioural contexts, but information on the sound production in this fish group is scarce. Here we examined the acoustic behaviour of the longsnout seahorse Hippocampus reidi by analysing sound production when fish were introduced to a new environment and during feeding, handling and courtship. We show that males and females produce two distinct sound types: ‘clicks’ (main energy between 50 and 800 Hz) during feeding and courtship, and previously undescribed ‘growls’ (main energy concentrated below 200 Hz).
The latter consists of series of sound pulses uttered in stress situations when the animals were handheld. Growls were accompanied by body vibrations, and may constitute an additional escape mechanism in seahorses, which might startle predators. During reproductive behaviour, clicks were most abundant on the third (last) day of courtship; they were particularly associated with the males’ pouch-pumping behaviour, suggesting synchronization between sound production and courtship behaviour. This is consistent with the biology of Hippocampus species, which are mostly monogamous and form pair bonds. Thus, a courtship call may be used to signal readiness to mate.